The perifornical hypothalamus (PFH) includes a large population of hypocretin (Hcrt) neurons intermingled with melanin-concentrating hormone (MCH) and other cell types. At present, there are no validated electrophysiological criteria to distinguish Hcrt cells in the PFH during extracellular recording in freely moving animals from other intermixed cell types. Recent electrophysiological studies performed in slices from mouse hypothalamus have shown that Hcrt cells can follow higher frequency electrical intracellular stimulation than adjacent non-Hcrt cells. Our pilot studies in anesthetized and freely moving rats revealed two subpopulations of hypothalamic cells which had different latencies and maximal frequencies of antidromic spikes during LC train stimulation. The goal of the proposed project is to obtain specific parameters of antidromic responses of Hcrt and adjacent cells, which will be labeled juxtacellularly in anesthetized rats with subsequent double immunostaining, to determine characteristics sufficient for identification of Hcrt cells in the freely moving animals. Proposed experiments will answer the following questions: 1) Do Hcrt, MCH and other perifornical cell types, which project to the LC, have specific electrophysiological characteristics? 2) What are the distribution of these Hcrt, MCH, and other cell subpopulations? 3) What are the discharge patterns of identified Hcrt and MCH cell subpopulations across the sleep-wake cycle? Our studies will lay the foundation for future studies and will provide the first data correlating behavior and discharge patterns of Hcrt and MCH cells. A better understanding of the physiological and behavioral role of these cell groups would have implications for development of effective treatments for obesity, depression, narcolepsy and other sleep disorders.